1. Field of Invention
This invention relates to soft electrical heaters, and particularly to heating elements, which have a soft and strong metal or carbon containing electrically conductive core.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed, for example U.S. Pat. No. 4,764,665 to Orban et al. This patent discloses an electrically heated fabric for use in gloves, airfoils and aircraft parts. In this patent the fabric is metallized after being formed in a glove structure, following weaving or arranging in a non-woven format. Copper bus bars are utilized for introduction of electrical current to the metallized textile. Having been made of a solid piece of fabric with metallized coating, this heating element doesn""t allow for flexibility in selection of desired power density. The metallizing of the formed heating element results in a loss of significant economies of scale, only a small number of embodiments can be achieved, thus severely limiting the potential application of this invention. The ""665 design is also not conducive to tight hermetic sealing through the heater areas (no gaps inside), which can cause a short circuit through puncture and admission of liquid into the body of heating element. This element can""t be used with higher temperatures due to the damage that would be caused to the polyaramid, polyester or cotton metallized fabric, described in the invention.
Another prior art example is U.S. Pat. No. 4,713,531 to Fennekels et al. Fennekels et al. discloses a sheet textile structure combined with resistance elements. These resistance elements comprise metallic fibers or filaments with a denier like that of natural or synthetic textile fibers, and with overall cross sectional thickness of 8 to 24 microns. The ""531 design suffers from the following drawbacks: being a sheet product, it is not conducive to hermetic sealing through the body of the heater (no gaps inside), only perimeter sealing is possible, which can result in a short circuit due to puncture and admission of liquid into the body of the heating element; yarns, comprising metal fibers, lack consistency of electrical resistance per given length, and their stretching, compression, or both, will result in very wide fluctuations in resistance, thus limiting the use of this technology in embodiments controlled by strict design and where an uncontrollable power output and temperature variability are unacceptable; yarns are very heavy: from 1 to 7 grams per 1 meter of yarn; the use of silver fibers makes these yarns very expensive; individual conductors have a large cross sectional thickness, each having a outer sheath of braided textile or elastomer.
Another prior art example is U.S. Pat. No. 4,538,054 to de la Bretoniere. The heating element of de la Bretoniere ""054 suffers from the following drawbacks: its manufacturing is complex requiring weaving of metal or carbon fibers into non-conductive fabric in a strictly controlled pattern; the use of the metal wire can result in breakage due to folding and crushing and it affects softness, weight and flexibility of the finished heater; it can not be manufactured in various shapes, only a rectangular shape is available; only perimeter sealing is possible (no gaps inside), which can result in a short circuit due to puncture and admission of a liquid into the body of the heating element; the method of interweaving of wires and fibers does not result in a strong heating element, the individual wires can easily shift adversely affecting the heater durability; the fabric base of the heating element is flammable and may ignite as a result of a short circuit; it is not suitable for high temperature applications due to destruction of the insulating weaving fibers at temperatures exceeding 120xc2x0 C.
U.S. Pat. No. 4,149,066 to Niibe at. al describes a sheet-like thin flexible heater made with an electro-conductive paint on a sheet of fabric. This method has the following disadvantages: the paint has a cracking potential as a result of sharp folding, crushing or punching; the element is hermetically sealed only around its perimeter, therefore lacking adequate wear and moisture resistance; such an element can""t be used with high temperatures due to destruction of the underlying fabric and thermal decomposition of the polymerized binder in the paint; the assembly has 7 layers resulting in loss of flexibility and lack of softness.
Another prior art example is U.S. Pat. No. 4,309,596 to George C. Crowley, describing a flexible self-limiting heating cable which comprises two conductor wires separated by a positive temperature coefficient (PTC) material. Said heating wires are disposed on strands of nonconductive fibers coated with conductive carbon. This method has the following disadvantages: (a) the wires are enveloped and separated by the tough PTC material which thickens and hardens the heating element (b) the distance between the wires is very limited, due to a nature of the PTC material having a high electrical resistance, this prevents manufacturing of heaters with large heat radiating surface; (c) the heater is limited only to one predetermined highest temperature level, therefore, this heating device is unable to bypass said temperature level when a quick heating at the highest temperature is needed.
The present invention seeks to alleviate the drawbacks of the prior art and describes the fabrication of heating element comprising metal coated, carbon containing or carbon coated threads, which is economical to manufacture; does not pose environmental hazards; results in a soft, flexible, strong, thin, and light heating element core, suitable for even small and complex assemblies, such as hardware. A significant advantage of the proposed invention is that it provides for fabrication of heating elements of various shapes and sizes, with predetermined electrical characteristics; allows for a durable heater, resistant to kinks and abrasion, and whose electro-physical properties are unaffected by application of pressure, sharp folding, small perforations, punctures and crushing.
The first objective of the invention is to provide a significantly safe and reliable heating element which can function properly after it has been subjected to sharp folding, kinks, small perforations, punctures or crushing, thereby solving problems associated with conventional flexible heating metal wires. In order to achieve the first objective, the electric heating element of the present invention is comprised of electrically conductive textile threads coated with metal, carbon, conductive ink, or their combination which possess the following characteristics: (a) high strength; (b) high strength-to-weight ratio; (c) very low coefficient of thermal expansion; (d) softness. The heating element core described in this invention is comprised of electrically conductive strips, sleeves, sheets, ropes, or strands of threads/fibers, which radiate a controlled heat over the entire heating core surface.
A second objective of the invention is to provide maximum flexibility and softness of the heating element. In order to achieve the second objective, the electric heating element of the invention contains thin (0.01 to 3.0 mm, but preferably within the range of 0.05-1.0 mm) threads, which are woven into, embroidered on, or stranded into continuous or electrically connected strips, sleeves/pipes, ropes, sheets, or bundles, then arranged and insulated to have gaps between the electrically conductive heating media. It is preferable that all insulation components of the heating element assembly are thin, soft and flexible materials.
A third objective of the invention is to provide for the uniform distribution of heat, without overheating and hot spots, thereby solving the problem of overinsulation and energy efficiency. In order to achieve this objective, (a) conductive threads in the heating elements are separated by non-conductive fibers yarns or polymers, (b) one side of the heating element may include a metallic foil or a metallized material to provide uniform heat distribution and heat reflection. It is also preferable that the soft heating elements of the invention are made without thick cushioning insulation, which slows down the heat delivery to the surface of the heating apparatus.
A forth objective of the invention is to provide for ease in the variation of heating power density, thereby solving a problem of manufacturing various heating devices with different electric power density requirements. In order to achieve the forth objective, the conductive threads/yarns in the heating element core are embroidered on, laminated between or woven into strips, ropes, sleeves/pipes, sheets, or stranded into bundles with predetermined width, density (of embroidering or weaving) and thickness. It is preferable that the strips, sleeves/pipes, sheets, ropes or strands are made of combination of threads/yarns with different electrical resistance and/or include electrically nonconductive high strength polymer or inorganic (such as refractory ceramic or fiberglass) fibers.
A fifth objective of the invention is to provide for ease in manufacturing of the heating element core, thereby eliminating a problem of impregnation of the whole fabric with stabilizing or filling materials to enable cutting to a desired pattern. In order to achieve the fifth objective, all strips, sleeves/pipes, sheets, ropes and threads are assembled into a desired stable shape prior to the heating element manufacturing.
A sixth objective of the invention is to provide a temperature self-limiting properties to the heating element core if dictated by the heater design thereby eliminating a need for thermostats. In order to achieve the sixth objective, the positive temperature coefficient (PTC) material is utilized in the selected areas of the heating element core.
The present invention comprises a heating element containing soft, strong and light electrically conductive threads/yarns acting as electrically conducting resistance heating media. The heating element is also highly resistant to punctures, cuts, small perforations, sharp folding and crushing. It can be manufactured in various shapes and sizes, and it can be designed for a wide range of parameters, such as input voltage, desired temperature range, desired power density, type of current (AC and DC) and method of electrical connection (parallel and in series). A heating element preferably consists of non-conductive fibers/yarns and electrically conductive metal containing or carbon containing threads/yarns woven into, embroidered on, laminated between or stranded into strips, ropes, sleeves/pipes, sheets or strands of threads.
The selected areas of the heating element core may contain highly conductive metal coated threads to provide redundant circuits in the heater. The heating element core may include a positive temperature coefficient (PTC) material to impart temperature self-limiting properties. The heating element core is shaped by folding or assembling of said conductive media into a predetermined pattern. The power supplying electrodes are attached to said heating element core and are electrically connected in parallel or in series. The soft heating element core is sealed to form an assembly containing at least one electrically insulating layer which envelops each strip, rope, sleeve/pipe, sheet or strand of threads.